1. Field of the Invention
The present invention relates to a method of resistive sheet transfer printing and an electrode head used in the field of image-forming technique for producing a high-quality image with high speed and sensitivity.
2. Description of the Prior Art
A high-speed production of a full-color image is suitably realized by a resistive sheet color transfer printing using a recording member (including an ink sheet having a resistive sheet carrying thereon ink containing a pigment or a sublimable dye and an image-receiving member having a color development layer in the surface thereof) and an electrode head. The electrode head has a multistylus thereof held by a plurality of insulating support members generally made of a thermosetting resin, glaze or ceramics such as alumina. The same material is used for both inside and outside of electrode pairs.
A resistive sheet transfer printing effected with a molten ink as a color material to realize a binary recording image at high speed, uses a film as a resistive sheet made of a polycarbonate resin containing carbon. This resistive sheet has a thermal diffusion coefficient of approximately 10.sup.5 m.sup.2 /s. Also, in order to reduce the contact resistance between the electrode head and the resistive sheet, a conductive film is deposited by evaporation or the like process as a second resistive layer on the surface of the resistive sheet (first resistive layer). According to a reference (KKC, TCU, Proceedings of the SID, 28/1, pp. 87 to 91, 1987), the contact resistance is expected to decrease by forming a second resistive layer of a Cr-N thin film having a specific resistivity of 0.03 ohm.multidot.cm or less and a thickness of 1000 .ANG. or less. The multilayered resistive sheet thus formed has a thermal diffusion coefficient of 10.sup.-6 m.sup.2 /s at most.
In the gradation recording using a sublimative dye as a color material for producing a high-quality full-color image, the high recording energy requirement poses the following problems in a conventional resistive sheet transfer recording system:
(1) When a resistive sheet of polycarbonate containing carbon is used in contact with an electrode head for recording, the low heat resistance and thermal sliding characteristic causes a smear on the head surface and deteriorates the image quality. In the case where a second inorganic-film resistive layer is deposited by evaporation, on the other hand, in spite of the decreased contact resistance, the especially inferior thermal sliding characteristic, combined with the failure to reduce the friction coefficient between the resistive sheet and the heads, still causes a head smear. This tendency is conspicuous especially for the relative-speed multiple recording system (which effectively uses a transfer member by delaying the running speed cf a transfer member as compared with the speed of a recording paper) and is accompanied by a considerable deterioration in the thermo-mechanical and electric characteristics of the resistive sheet.
(2) In the case where the electrode head is configured of a stylus electrode and a common electrode in opposed relationship to each other to record a signal current in parallel to a heat-generating substrate, the current density distribution is concentrated in the vicinity of the stylus and therefore large homogeneous recording dots are not obtained, thereby making the system unsuitable for gradation recording.
(3) The thermal diffusion coefficient of the insulating support member of the head and the resistive sheet is not optimized. Nor are high speed and high sensitivity attained taking heat storage control into consideration.
If an insulating support member small in thermal diffusion coefficient is used for the electrode head, sensitivity would be improved but the color of a recorded image would become less clear and the resolution thereof would be reduced due to heat storage. The use of an insulating support member large in thermal diffusion coefficient, by contrast, would deteriorate the sensitivity at the sacrifice of the features of resistive sheet transfer printing. Further, heat pulses generated as a result of applying a signal current to the electrode pairs are concentrated in the vicinity of the electrodes of the resistive sheet. This makes it impossible to produce homogeneous recording dots and causes a corrosion of the train of positive electrodes.